Methylcobalamin (methyl-B12, vitamin B12 analog) at a concentration of 0.2 x 10(-5) M tended to inhibit the toxic effect of 1 and 1.5 x 10(-5) M methylmercuric chloride (MMC) on the development of nerve fibers, glial cells and fibroblasts from newborn rat cerebellum explants. Methyl-B12 at greater than 1 x 10(-5) M significantly inhibited the toxic effect of 1 and 1.5 x 10(-5) M MMC. However, the protective effect of methyl-B12 against the toxicity of mercuric chloride (MC) was not significant. These results are in accord with an interaction of alkyl mercurials with membranes of nervous tissue, producing the degenerative changes in the cells, since vitamin B12 increases the lipid synthesis in nervous tissue. The results also indicate that the mechanism of toxic action of organic mercury is different from that of inorganic mercury.